The present invention relates to a diversity receiving system for use in reception of a radio signal transmitted from a transmitter in a digital radio communication, and, in particular, to such a diversity system which enables to select a proper branch by estimating propagation path property by use of a training signal so as to reduce intersymbol interference caused by multipath selective fading.
In a communication system, especially, a mobile communication system, an input signal level for a receiver varies intensely by variation in a propagation path and/or a transmission medium of a radio signal. This phenomenon is known as a fading.
In order to avoid a bad effect from the fading, a diversity receiving system of various types is also known in the prior art. The diversity receiving system usually comprises a plurality of receiving branches which produces branch outputs, respectively. A resultant output of the diversity receiving system is obtained as a composite signal of the branch outputs or as one selected from the branch outputs.
When a transmitter transmits a transmission signal as the radio signal, a part of the radio signal directly propagates to a receiver. However, a remaining part is reflected and scattered by various bodies such as buildings, mountains and the like and therefore, reaches to a receiving system with a delay.
Accordingly, the input signal received at the receiving system comprises the direct signal component and the delay component and suffers from distortion. The propagation path causing such a distortion is known as a multipath and the distortion is referred to as a multipath distortion.
In one method for reducing the multipath distortion, the radio signal transmitted from the transmitter comprises a training signal and a data signal. The receiving system estimates the transmission property of the multipath by use of the training signal and equalizes the received signal by the estimated signal to reduce the multipath distortion.
When the diversity receiving system is used for the multipath, the multipath distortion is different at receiving branches. Therefore, when the branch output selection is carried out by, for example, the input signal intensity for each branch, the resultant signal from the diversity receiving system is affected by a different transmission property of the multipath.
A diversity receiving system for effectively reducing the multipath distortion is disclosed in a Japanese Patent Application No. 113202/1989 filed on May 2, 1989 by the same assignee (NEC Corporation). The corresponding U.S. patent application was filed by the applicant, Kazuhiro Okanoue, on May 2, 1990 whose serial number is Ser. No. 07/517,883 and assignee is the same as the present application. The disclosed diversity receiving system uses the training signal in the transmission signal and an equalizer for equalizing the resultant signal. In the proposed diversity receiving system, a propagation property is estimated at each receiving branch by use of the training signal received thereat. The estimated propagation path property is classified into components significant and insignificant for the equalizer. A ratio of a power of the insignificant components and another power of the insignificant components are calculated. The ratio in each branch is used as a parameter for selecting one of the branches. That is, the ratios from all of the receiving branches are compared with one another and one of the branches providing the largest one of the all ratios is selected as a specific branch. Then, the branch output from the specific branch is selected as the resultant signal of the diversity system and is equalized by the propagation path property estimated in the specific branch. Therefore, equalization is effectively carried out and the receiving performance is improved by the diversity effect.
However, when the training signal has a predetermined bit length, for example, 26-bit length, the data signal affects estimation of the propagation path property using the training signal, so that an error is included in the calculated ratio, especially, the power of the insignificant components. This results in degradation of S/N ratio in branch selection. The degradation of S/N ratio is considerably observed when the intersymbol interference is caused within an equalizing performance of the equalizer.